1. Field of the Invention
The present invention relates to the field of computer animation and, in particular, to skin and flesh simulation using finite elements, biphasic materials, and rest state retargeting.
2. Description of the Related Art
Computer-based animations often involve modeling the behavior of flesh and skin on characters and other objects such that skin and flesh move in a believable and visually pleasing manner when the characters are animated. The movement of skin and flesh is simulated by using a physics-based simulation software application. Such a software application determines the position of a character's skin and flesh based on the movement of the character and the model used for the flesh and skin. One approach for modeling flesh involves a material with a specific stiffness, such that flesh stretches as the underlying character moves and deforms. The animator selects a proper stiffness that causes the character's flesh to move in a plausible manner as the character undergoes various basic movements, such as walking, crawling, or running.
One drawback of this approach is that, while such a model may provide plausible flesh movement when the character undergoes movement involving small deformations, the model behaves poorly when the character undergoes larger deformation. For example, a stiffness value could be selected to creates plausible flesh movement for low deformation actions such as walking. The same stiffness value may cause the character's flesh to separate, or rip away, from the character if the character undergoes a rapid change in motion, such as jumping off a bridge. As a result, animators may need to manually adjust each shot to achieve plausible flesh motion, particularly when a character undergoes a sudden change in momentum. For example, an animator could add an inertial field generator associated with the character, so causing the simulator to believe the character is not actually jumping off a bridge. Such manual per-shot approaches are time consuming for the animator, and thus increase production costs and production cycles.
Skin motion may be simulated by using a mass-spring system. With such an approach, discrete nodes in space are each assigned a mass. The nodes are interconnected via springs, where the springs are models for forces that allow the skin to deform and then to restore to an original position as the underlying character animates. The interconnected nodes form a mesh that surrounds the character. One set of springs attempts to maintain the relative positions of the masses by maintaining the length of the edges connecting the masses. Potentially a second set of springs attempts to maintain the areas of the triangles formed by the mesh that interconnects the nodes. Typically, such mass-spring models exhibit linear stretching behavior. One drawback of this approach is that each spring in mass-spring systems typically attempts to maintain the spring's initial, or reference, length, resulting in an incorrect visual appearance of the skin or flesh as the animated character experiences sudden or exaggerated motion. Another drawback of this approach is that such mass-spring models can cause the skin to stretch unrealistically across portions of the character while compressing and folding over in others portions of the character. As a result, such mass-spring models typically involve multiple simulation parameter sets for different classes of motion, resulting in increased simulation setup time for each different shot.